Currently there exist several different types of braking systems for car trailers, but for trailers with a total weight up to 3500 kg, overrun brakes are the most common. When a car with attached trailer is braking, the trailer will be pressed against the tow bar of the car and the retardation force will be absorbed by the trailer's ball hitch attachment which is mounted on a slide. This retardation force will move the slide, which transfers the force and movement to the trailer's brake system via mechanical rods and wires or via a hydraulic system. There are also brake systems which are driven by electricity or compressed air.
In order to have the brake force transmitted gradually to the brakes and preventing the wheels from being locked, most trailers with overrun brakes have a gas spring acting as a shock absorber on the slide. This gas spring is also pushing the slide out again after each braking sequence
Mechanical overrun brakes, which is most commonly used in Europe, has a mechanical overrun device and transferring of brake forces to drum brakes via rods and cables. These are simple constructions and work well during forward driving, but they have a great problem in that the brakes are also activated during reversing. Several different systems have been designed to limit or prevent the braking effect during reversing, but they all have various shortcomings and unwanted features.
Currently most trailer manufacturers in Europe are using the German “Rücknnatic” or similar systems, which make the brake shoes release some of the pressure against the drums when wheels are rotating backwards. These systems make reversing heavy because the brakes are not deactivated 100% and if not maintained and adjusted correctly backing up may be impossible because of locked wheels.
Another unwanted and dangerous feature with these systems is that the hand brake only works when trailer is rolled forward. This means the trailer cannot be parked with front end uphill without putting backstops behind the wheels.
When heavy boats are rolled on/off boat trailers in steep ramps accidents have happened repeatedly because of sudden loss of brake force. With Rücknnatic and similar systems the hand brake will only keep wheels locked up to a certain point of torque, when gravity is pulling a trailer backwards. When gravity force exceeds the braking torque of the hand brake, the brake shoes are losing grip the same way as during reversing and the wheels will suddenly start rolling backwards.
To prevent the braking effect 100% when reversing with overrun brakes, a lot of mechanical solutions for locking up the slide has been made over the past years.
As an example, a spring-loaded steel lever is manually put into a slot before reversing and when driving forward again the spring pushes the lever aside and the slide is free to move again. This solution incurs a safety risk as it may fasten in the locked position due to insufficient lubrication and maintenance. In addition, this is a mechanical device where the user must get out of the car to activate manually, as opposed to the inventive system which may be activated automatically when the gearchange lever is put into reverse.
Other known devices for preventing the braking effect during reversing with mechanical overrun brakes are for example patents GB1341724 and GB2385102. These are based on the principle of inserting a hydraulic cylinder with a piston as a part of the brake rod. A solenoid controlled valve may open or close for hydraulic fluid to pass from one side of the piston to the other. The principle here is that the piston shall be locked against a hydraulic pressure for the braking system to work during driving, and that this pressure ceases during reversing because a solenoid controlled valve is opened. A safety weakness of this principle is that the braking effect is reduced or ceases completely if a leak occurs in the hydraulic system of the cylinder.
Hydraulic overrun brakes, or surge brakes, which are more common in the USA, have also a gas spring mounted as a shock absorber in parallel with the master pump, but braking action during reversing is usually prevented by a solenoid controlled valve which closes the flow of hydraulic fluid from the master cylinder. Even though such hydraulic surge brake systems have solved the problem of braking effect during reversing, the master cylinder has to carry all the compressive force required to back the trailer. Backing a full loaded trailer up a steep hill may result in overloading of the master cylinder and the brakes may fail if the master cylinder or the hydraulic pipe/hose system should develop a leak.